1. Field of the Invention
The invention relates to an automatic focusing device, for use in a picture pickup device such as a camera, which detects the focus state of the objective lens by receiving light reflected back from a subject through the lens and keeps the lens in focus.
2. Description of the Related Art
Conventionally, a focus detecting technique has been developed which forms two images by reimaging beams of light reflected back from a subject that pass through first and second areas, respectively, of the objective lens which are symmetrical in position with respect to the optical axis and seeks the positional relation between the two images, thereby obtaining a displacement of a position in which the two images are formed by the objective lens from the predetermined position of its focus and the direction of the displacement, i.e., information as to whether the imaging position of the objective lens is in front of or at the rear of the predetermined focus position.
FIG. 16 shows an arrangement of the optical system in a focus detecting device based on such a conventional technique. The optical system includes a predetermined focal plane (position) 111 at the rear of an objective lens 38, a condenser lens 36 behind the plane 115, and reimaging lenses 35L and 35R behind that condenser lens. Image sensors 34L and 34R are placed on the image formation planes of the respective reimaging lenses 35L and 35R.
As indicated at 116L and 116L in FIG. 17, the images of a subject on the respective image sensors 34L and 34R draw to the optical axis 100 in the case of the so-called front focus condition in which a subject to be brought into focus is imaged in front of the predetermined focal plane 115. In the so-called rear focus condition, on the other hand, they are away from the optical axis 100. When the subject is brought into focus, the spacing between two corresponding points each of which is on a respective one of the images formed on the sensors 34L and 34R reaches a specific distance which is uniquely defined by the arrangement of the optical system in the focus detecting device, i.e., the in-focus time spacing.
In principle, therefore, the state of focus will be known by detecting the spacing between two corresponding points on the two images. The spacing between the two corresponding points can be detected by seeking correlation between two light intensity distributions on element areas of the image sensors 34L and 34R, shifting the position of the element area of one of the images sensors 34L and 34R with respect to the other, and seeking the spacing between the element areas of the image sensors for the best correlation is obtained.
With an automatic focus adjusting device having such a focus detecting optical system built in, the seeking of the shifted position for the best correlation, calculation of the spacing between the two images, calculation of the amount of defocus indicating the state of focus, calculation of the amount by which the lens is to be driven, and driving of the lens are performed under program control of microcomputer-based control means.
In addition, a focus detection related technique is disclosed in Japanese Unexamined Patent Publication No. 1-187521 which detects the state of focus of the objective lens by receiving light reflected back from a subject through the objective lens. In this technique, when the focus detection is impossible, a low contrast scan operation is performed to search for the lens location that enables the focus detection while the lens is driven. That is, according to this technique, when it is decided that the focus detection is impossible, the range over which the focus detecting operation is performed with the lens driven is limited to a narrow one, thereby reducing the time it takes to make a decision that the focus detection is impossible.
Moreover, in Japanese Unexamined Patent Publication No. 62-187829 is disclosed an automatic focus detecting technique for use in a camera in which a plurality of focus detecting areas of different sizes are set up so as to make a subject easy to pick up and the focus detecting areas are switched by a photographer on his judgment of the condition of the subject. That is, when an object is present in a scene to be shot that obstructs the focus detecting operation, a focus detecting area in the shape of a small spot is selected, while, when it is desired to shoot a moving subject, a large focus detecting area is selected, thereby improving the accuracy of focus detection.
Furthermore, a focus detecting technique is disclosed in Japanese Unexamined Patent Publication No. 63-17418, which, at first, detects focus using a small focus detecting area and, when a focus undetectable state is detected, uses a larger focus detecting area. This technique improves the accuracy of the focus detection as in shooting a moving subject by making the focus detecting area variable.
However, the conventional focus detecting techniques described above each have a range of defocus amount over which the focus is detectable which depends on the arrangement of their optical system.
The focus detectable defocus range varies according to the size of a focus detecting area on focus detecting photosensitive elements even with the same optical system used.
This will be described below.
FIG. 18 shows an example of setting up focus detecting areas on the image sensors 34L and 34R and seeking the relative position of those areas in which the best correlation between light intensity distributions on the sensors is obtained.
In the upper half of FIG. 18, a focus detecting area a1 is fixed on the image sensor 34L and a focus detecting area a2 is shifted on the image sensor 34R and correlation between light intensity distributions on the focus detecting areas a1 and a2 is taken for each shifted position. In this case, the spacing between the focus detecting areas a1 and a2 when the area a2 is shifted to the position in which the best correlation is obtained is sought. The magnitude and sign of the difference between that spacing and the in-focus time spacing indicate the amount and direction of focus displacement, respectively.
The lower half of FIG. 18 shows the case where focus detecting areas b1 and b2 which are respectively larger than the focus detecting areas a1 and a2 are set up.
In the case of the focus detecting areas a1 and a2, the focus-detectable range of defocus amount is 12a-11a. In the case of the focus detecting areas b1 and b2, the corresponding range is 12b-11b. Both the ranges are related such that EQU (12b-11b)&lt;(12a-11a)
It can therefore be said that the focus-detectable range of defocus amount for the focus detecting area b is smaller than that for the area a.
On the other hand, the amount of defocus of an objective (photo-taking) lens varies according to the distance between a subject and the lens over the range from the closest focusing distance to infinity. In general, the longer the focal length of an objective lens, the larger its maximum range of defocus becomes. If the defocus amount of an objective lens is larger than the focus detectable defocus range at the time of shooting, the focus detection will become impossible.
In the above-described technique disclosed in Japanese Unexamined Patent Publication No. 63-17418, at first the focus detection is performed through a small focus detecting area and, when the focus detection is impossible, a larger focus detecting area is used. As described above, when a large focus detecting area is used, the focus detectable defocus range decreases. Thus, when the defocus range of an objective lens is so large as to exceed the focus detectable defocus range for a small focus detecting area, the focus detection becomes impossible. Further, even if a large focus detecting area is used, the focus cannot be detected. A time delay involved in displaying on the viewfinder or the like that the focus detection is impossible further increases, thus making photographers feel strange. In addition, the low contrast scan operation performed when the focus detection is impossible would further increase the time delay.
Further, when a subject is in low light, a problem arises even if the low contrast operation is performed in that the focus detection is substantially impossible because the integration time required for the amount of charge stored in the image sensor to reach a level suitable for focus detection increases. When a subject is in lower light and the use of assist light is required for focus detection, the focus detection is impossible if the amount of defocus of an objective lens exceeds the focus-detectable amount of defocus with the first focus detecting areas. In this case, even if the second focus detecting areas larger than the first ones is used for focus detection, the focus detection is still impossible because the focus-detectable amount of defocus is exceeded, resulting in a useless operation and a time lag.